Leaven

A leaven /ˈlɛvən/, often called a leavening agent /ˈlɛvənɪŋ/ (and also known as a raising agent), is any one of a number of substances used in doughs and batters that cause a foaming action (gas bubbles) that lightens and softens. An alternative or supplement to leavening agents is a mechanical action by which air is incorporated. Leavening agents can be biological or synthetic chemical compounds. The gas produced is often carbon dioxide, or occasionally hydrogen, or another.

When a dough or batter is mixed, the starch in the flour and the water in the dough form a matrix (often supported further by proteins like gluten or polysaccharides, such as pentosans or xanthan gum). Then the starch gelatinizes and sets, leaving gas bubbles that remain.

Chemical leavens are mixtures or compounds that release gases when they react with each other, with moisture, or with heat. Most are based on a combination of acid (usually a low molecular weight organic acid) and a salt of bicarbonate (HCO₃⁻). After they act, these compounds leave behind a chemical salt. Chemical leavens are used in quick breads and cakes, as well as cookies and numerous other applications where a long biological fermentation is impractical or undesirable.

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